The present invention relates to a method and apparatus for smelting iron ore in which the iron ore is preheated and partly reduced within a first reaction zone and then further reduced and melted in a second reaction zone to produce a molten iron product. More particularly, the present invention relates to such a method and apparatus in which a carbonaceous feed is heated within a third reaction zone to produce hot char that is fed into the second reaction zone together with an oxidant. Even more particularly, the present invention relates to such a method and apparatus in which evolved volatile substances produced during char production are oxidized to produce a heating gas that is introduced into the first reaction zone in order to preheat the iron ore.
Coal is generally the most common and cost-effective carbonaceous material for the production of iron. A blast furnace is, by far, the most common, thermally efficient, and best established method to use coal to produce molten iron. However, appropriate coal must first be converted to metallurgical coke in a coke oven prior to introduction into the blast furnace. There are two practical problems with this approach. One problem is that an appropriate coal may be unavailable or prohibitively expensive in some parts of the world. A more serious problem is that toxic liquid and fugitive gas products are emitted from the coke plant that can cause significant environmental issues. As a result, there has been a world wide effort to develop coal-based iron production processes that can use coal directly to produce iron with substantially reduced gaseous or liquid pollutant emissions.
These coal-based iron production processes generally have two reaction zones. A first reaction zone preheats and partially reduces the ore. A second reaction zone contacts coal with an oxidant, usually substantially pure oxygen, to reduce and melt iron, slag, and produce a reducing gas, which is fed to the first reaction zone to effect the partial reduction of the ore. Hydrogen and CO are the active components for this reducing gas. There are, at least, a half dozen coal-based iron production processes under development that conform to the above generic process description and have unique features within this framework. The COREX process, developed by Vorst Alpine Industrieanlagenbau GmbH is the only fully commercialized coal-based iron production process.
As a group, these coal-based processes are less thermally efficient than the conventional blast furnace process for iron production. The thermal efficiency of these coal-based processes are primarily limited by the performance of the second stage. The second stage has two functions: compete reduction of the iron oxide ore to elemental iron and melt the iron and slag products. Iron ore reduction and melting is highly endothermic. The energy required for this reduction is most cost-effectively supplied by partial oxidation of coal. However, in order to produce molten iron, the CO/CO.sub.2 and H.sub.2 /H.sub.2 O molar ratios must be very high, on the order of 20/1. On the other hand, oxidation of carbon to CO.sub.2 provides roughly four times the energy of oxidation carbon to CO. Worse yet, H.sub.2 production from coal is highly endothermic. As a result, there have been numerous attempts to minimize the hydrogen content of the coal feed and use the energy of oxidation of carbon to CO.sub.2 without oxidizing the iron product.
U.S. Pat. No. 5,613,997 discloses a technique to increase the thermal efficiency of the second reaction zone of a generic coal-based iron ore production processes based on the fact that coal has three broad classifications of components: volatile matter, fixed carbon, and ash. The ash is an undesirable component that should be minimized through the coal selection process. The volatile matter is a class of hydrocarbons that contains most of the hydrogen in the coal. The fixed carbon is a class of hydrocarbons that contains the highest concentration of carbon and is the most desirable feed for second reaction zone. In this patent, a third reaction zone reactor is used to preheat the fixed carbon and ash components of the coal prior to feeding into the second zone by contacting the coal with an oxidant to selectively and efficiently oxidize a portion or all the volatile matter to produce gaseous combustion products with high CO.sub.2 /CO and H.sub.2 O/H.sub.2 ratios and efficiently transfer this heat of combustion to the fixed carbon, ash, and any remaining volatile matter in the coal to produce a hot char feed for the second zone of the generic coal-based iron production process. The foregoing has increased the thermally efficiency of the second reaction zone to the point that the thermal efficiency of the first reaction zone limits the overall process efficiency for, at least, some of the generic coal-based iron production processes.
As will be discussed, the present invention provides methods and apparatus to increase the thermal efficiency of the first reaction zone so that even higher thermal efficiencies may be realized than were possible in the prior art. The increase in thermal efficiency can be used to increase throughput.